Accurate measurement of the volume flow of the breathing gas, e.g., within the flexible breathing tube, is of particular interest in respiration systems. The velocity of flow of the fluid is the relevant variable to be measured here for the determination of the volume flow of a fluid through a known conduit cross section.
Hot-wire or hot-film anemometers are used in many cases to measure the velocity of flow of a gas used for respiration. These are associated with the advantage that they do not represent any appreciable flow resistance themselves. A hot-wire or hot-film anemometer comprises a thin heated element, the so-called hot wire or hot film, which is cooled by the fluid flowing past it. The resistance of the wire or film is measured, which itself depends on the temperature. Depending on the value of the velocity of flow of the fluid flowing past, the hot wire or hot film is cooled more or less intensely, so that the resistance is an indicator of the velocity of flow of the fluid flowing past.
Another advantage of these elements is the small thermal mass of the measuring element itself. The measurement is extensively independent of the temperature when the temperature of the hot wire or hot film is more than 250° C. above the fluid temperature, and fluid temperature measurement and compensation can now be eliminated. For applications with overtemperatures of less than 150° C., an additional “cold wire” is needed to compensate changes in the gas temperature. The “hot wire” and the “cold wire” are connected to this end in a Wheatstone bridge, and the hot wire is adjusted to a constant overtemperature relative to the cold wire, so that the heating current being fed depends directly on the amount of heat removed per unit of time with the fluid flow. This conventional operation of the hot-wire or hot-film anemometer is called constant-temperature anemometer mode (CTA mode).
However, besides the volume flow of the fluid flowing past, the absolute temperature of the fluid relative to the hot wire or hot film as well as the composition of the fluid and hence the thermal conductivity and heat capacity of the fluid also affect the extent of cooling, so that these parameters must be taken into account, in principle, when measuring the velocity of flow, if they are subject to relevant changes.
A device and a process for measuring a volume flow difference between the inspiratory volume flow and the expiratory volume flow by means of temperature anemometry are known from DE 10 2005 000 964 B3.
DE 196 47 350 A1 discloses a device and a process for measuring the volume flow of gases. The run time of a thermal signal between two measuring means along the direction of flow of the gas is determined here.
It is described in WO 01/18500 A1 (corresponding to U.S. Pat. No. 5,339,687) how a mass flow measurement is obtained with a heating element and two temperature sensors from the temperature signals of the temperature sensors taking into account measured, substance-specific characteristics to characterize the heat transmission characteristic.
It is especially important in respiration technique to measure the changes in the velocity of flow of the breathing gases accurately and especially rapidly. The problem was found in this connection that, for example, hot-film sensors based on planar semiconductor chips have relatively slow response times and above all slow fall times due to the storage of the amount of heat during rapid changes in the velocity of flow. While this problem can still be partially eliminated by a rapid adjustment and by a rapid rise of the heating current in case of a sudden rise in the velocity of flow (e.g., from 0 L/minute to 100 L/minute) in the CTA mode, the fall time will be 10 times higher than the rise time in case of an abrupt drop in the velocity of flow (e.g., from 100 L/minute to 0 L/minute). This is due to the fact that the regulator can only switch off the element but it cannot cool it. Switching off the heating for the element is not, moreover, meaningful, because there must be a potential difference on the bridge amplifier for a restart of the bridge circuit.
For example, sensor chips for measuring the velocity of flow of a fluid, in which two resistive heating elements and two resistive gas temperature elements are united on one membrane, are known from the Fraunhofer-Institut für Siliziumtechnologie (Fraunhofer Institute of Silicon Technology) in Itzehoe (ISIT). A heating element and a precision resistor each are connected in a gas temperature-compensating Wheatstone bridge in the CTA mode. The two bridges operate electrically fully autarchically. However, the heat transfer from one heating element to the other heating element is used as information due to the use of two heating elements, similarly to the device disclosed in DE 196 47 350 A1, because, due to the heat transfer, less heating current is needed in the heating element located downstream to maintain an overtemperature predetermined in the CTA mode. It is thus possible to recognize the direction of flow as long as the heat transfer does not drift at high velocities of flow of the fluid. The bridge located downstream is thus used as a direction indicator for the fluid flow, while the bridge that is located upstream and is in the CTA mode determines the amount of the velocity of flow. As an alternative, it is also possible to use the amount of the heat transfer from the heating element located upstream to the heating element located downstream as a difference of the heating current values needed as a measured value of the velocity of flow. However, the losses due to heat transfer which are caused by drifting are problematic at high velocities of flow.
However, the problem of the flow fall times is not solved by any of the prior-art devices nor by any of the prior-art processes. Furthermore, it is a fundamental problem of the hot-wire and hot-film anemometers that the characteristic describing the relationship between the velocity of flow and the heat transfer depends on the thermal conductivity properties of the fluid, i.e., on the thermal conductivity and heat capacity of the fluid, which are determined, among other things, by the composition, temperature and pressure of the fluid. The measurements must therefore be corrected by means of external information, e.g., concerning the fluid composition, in case of the prior-art devices and processes, or calibrated for certain fluids. Changes occurring in the fluid composition during the measurement cannot, in general, be taken into account without additional aids, and they thus distort the measurement. This is a special problem especially in respirators.